Six underground tanks that hold a brew of radioactive and toxic waste at the nation's most contaminated nuclear site are leaking, Gov. Jay Inslee said Friday.

The leaks raise new concerns about delays for emptying the tanks at south-central Washington's Hanford nuclear reservation, and they strike another blow to federal efforts to clean up a site plagued by delays and budget overruns.

State officials just last week announced one of Hanford's 177 underground tanks was leaking in the range of 150 to 300 gallons a year, posing a risk to groundwater and rivers. So far, nearby wells haven't detected higher radioactivity levels.

Inslee traveled to Washington, D.C., this week to discuss the problem with federal officials. He said he received the "very disturbing news" during meetings Friday that six tanks are leaking.

The tanks, which already are long past their intended 20-year life span, hold 53 million gallons of highly radioactive waste - enough to fill dozens of Olympic-size swimming pools - left from decades of plutonium production for nuclear weapons.

The federal government created Hanford in the 1940s as part of the top-secret project to build the atomic bomb, and the site went on to produce plutonium for the nation's nuclear weapons arsenal for years.

Today, it is the country's most contaminated nuclear site, with cleanup expected to last decades and cost billions of dollars.

Central to cleanup is the construction of a plant to convert the waste into glasslike logs for safe, secure storage. The $12.3 billion plant is billions of dollars over budget and behind schedule.

Inslee and Oregon Gov. John Kitzhaber have championed building additional tanks to ensure safe storage of the waste until the plant is completed.